DAVID BIANCULLI, HOST:

This is FRESH AIR. I'm David Bianculli, editor of the website TV Worth Watching, sitting in for Terry Gross. Today's guest wants you to get more sleep, and he has some warnings about how not getting enough sleep can affect your concentration, memory, immune system, blood sugar levels, appetite and more. On the bright side, he has some suggestions to help you get and stay asleep. Matthew Walker directs the Center for Human Sleep Science at the University of California, Berkeley, where he's a professor of neuroscience and psychology. In his sleep lab, he's conducted many sleep studies using the latest techniques to image the brain and monitor brain activity in order to investigate why we sleep and what happens when we sleep and when we dream. He's currently investigating the possible link between inadequate sleep and the development of Alzheimer's disease. His book, which is now in paperback, is called "Why We Sleep." Terry spoke with him last year when it was first published.

(SOUNDBITE OF ARCHIVED BROADCAST)

TERRY GROSS, BYLINE: Matthew Walker, welcome to FRESH AIR It's a pleasure to have you here. So something unusual happened. I usually wake up like once or twice in the middle of the night. I've done that all my life. I've never understood how people can actually lie down and then just wake up the next morning without even having moved all night. But I slept till 5:30, which was kind of shocking, and then I had a great deal of difficulty falling back to sleep 'cause my alarm was set for 6:50. So on the one hand, I slept really well. And on the other hand, I didn't (laughter). Like, before 5:30, great. After 5:30, not great. So how did I...

MATTHEW WALKER: Well (laughter)...

GROSS: You tell me. How did I sleep last night?

WALKER: So it depends on how long you were staying awake after that 5:30 morning awakening. And the advice, usually, by the way, is this - that you should not actually stay in bed for very long awake because your brain is this remarkably associative device. And it quickly learns that the bed is about being awake. So you should go to another room - a room that's dim. Just read a book. No screens. No phones. And only when you're sleepy, return to the bed. And that way your brain relearns the association with your bedroom being about sleep rather than wakefulness. So even if you have, perhaps, not had quite enough sleep, I promise you that I will diplomatically not point out any of the cognitive failures that I am hearing through the headphones as we speak.

GROSS: Oh, no (laughter). You can tell I didn't sleep.

(LAUGHTER)

WALKER: I will say nothing from this point forth.

GROSS: But here's the thing, and I'm sure everybody's experienced this. It's like you have maybe an hour, an hour and a half left to sleep. The clock is ticking. You know the time is running out. And if you don't fall asleep now or soon, it's going to be too late. But if you get up, you're automatically saying, I am sacrificing 10 to 20 minutes of the little amount of time I have left to sleep. I'm just going to, like, write that off. It would just make me feel utterly defeated and make me more anxious about not being able to sleep.

WALKER: So what you're describing is that fearful rolodex of anxiety that happens when people wake up. You know, you start thinking, oh, my goodness, I've only got an hour and a half left on the clock, and then I've got to be up. I've got this long day ahead of me. Another thing that people can do if you don't want to get up and go to a different room is actually try meditating. It turns out that - being quite a stoic scientist, a hard-nosed scientist, I actually didn't really believe the data even though the data is very strong. And I started doing it myself, particularly when I was traveling with jetlag. And I found it to be very effective. And it just quiets the mind, and it dampens down what we call the fight-or-flight branch of the nervous system, which is one of the key features of insomnia. And that can really have some efficacious benefits, too. So that's another solution if people would choose not to go to a different room.

GROSS: How did you sleep last night?

WALKER: I actually slept quite well. I got back from a period in the U.K. last week, so I'm just starting to come to the tail end of my jetlag. But I had a very good night of sleep, had - I always give myself a non-negotiable eight-hour opportunity. And I clocked around about - probably about seven hours and 45 minutes.

GROSS: Eight hours in bed every night - how do you do it?

WALKER: Well (laughter)...

GROSS: I'm like 6 1/2 to seven hours. How many hours of sleep should I be getting?

WALKER: So the recommendation by the World Health Organization is eight hours, and most people don't put their head on the pillow and instantly fall asleep. It's not like a light switch. Falling asleep is like landing a plane. It takes time. You've got to sort of gradually descend. I think one of the problems with insufficient sleep is people are not very good at predicting how poorly they are doing when they are under-slept. So your subjective sense of how well you're doing is a miserable predictor of objectively how you're doing. So it's a little bit like a drunk driver at a bar. They've had a couple of shots and some beer. And they stand up, and they say, well, I'm perfectly fine to drive home. And you say, no, I know that you think you're fine to drive. But trust me. Objectively, you're not, and the same is true for sleep. So I think many people walk through their lives in an under-slept state not realizing it. It's become this new natural baseline.

GROSS: Yeah, but most people don't have eight hours available to sleep. So if you're sleeping, say, 6 1/2 to seven hours, as I think a lot of adults do, how much are you depriving yourself of? I mean, what are you losing? What are your deficits because you're not sleeping enough?

WALKER: Well, I think the first general point to make from epidemiological studies across millions of people is the following - that short sleep predicts a shorter life. It predicts all cause mortality. So that sort of classic maxim that you may have heard, that you can sleep when you're dead - it is actually mortally unwise advice from a very serious standpoint. We also know that every disease that is killing us in developed nations has causal and significant links to a lack of sleep. And a lack of sleep defined as six hours of sleep or less. So I think people really need to start to, I think, become much more aware of the science of sleep. And I think that, in part, is why people like me have failed. We've not done a good job at communicating the science and the impact of insufficient sleep to the public.

GROSS: I just lost my train of thought for a second. I think I needed more sleep.

(LAUGHTER)

WALKER: I'm saying nothing, Terry. I am saying absolutely nothing.

GROSS: So I cheat on sleep during the week, and only get like 6 1/2 to seven hours. But on the weekend, I'll try to sleep like eight hours sometimes. Sometimes I'm in bed for nine hours, which is, like, I think, amazing. So am I making up for the deficits of the week?

WALKER: So you're trying to sleep off a debt that you've lumbered your brain and body with during the week. And wouldn't it be lovely if sleep worked like that? Sadly, it doesn't. Sleep is not like the bank. So you can't accumulate debt and then try and pay it off at a later point in time. And the reason is this - we know that if I were to deprive you of sleep for an entire night - take away eight hours - and then in the subsequent nights, I give you all of the sleep that you want - however much you wish to consume - you never get back all that you lost. You will sleep longer, but you will never achieve that full eight-hour repayment as it were. So the brain has no capacity to get back that lost sleep that you've been sort of lumbering it with during the week in terms of a debt.

GROSS: Do people of ages have different biological clocks - in the sense that young people seem to like to go to bed very late and then sleep very late? As people get older, they tend to go to bed earlier and get up earlier. Is that something biological?

WALKER: It is biological. And what you're describing is our 24-hour rhythm or what we call the circadian rhythm. And it undergoes this sort of dramatic set of changes across the lifespan. Sort of early in life when we're children, despite wanting to stay up late, we find it difficult because we go to bed early. Then we wake up early. As we shift into adolescence and that teenage period, now that 24-hour clock shifts forward in time. So you want to go to bed late and wake up late. And then gradually, it stabilizes into adulthood. And then as you progress with age, it starts to regress back again. So you start to go to bed earlier and wake up earlier. There is variability, however, from one individual to the next. And that is actually genetically pre-determined. It's called your chronotype, and another way of saying this is that you may be an owl or you may be a lark. So you may be someone who likes to stay up late and then wake up later in the morning. Those would be owls. And the lark - the opposite - they're the early risers, and they are the early-to-bed people. And about 30 percent of the population is one of those two extremes. And then the rest of us sort of sit somewhere safely in the middle.

GROSS: It's interesting that that's biological. And you can't say to somebody, oh, you're just staying up so late; you should be getting up earlier. They might not be wired to do that.

WALKER: They aren't. And it's not their fault. It is their genes. And I think we do a disservice to the owls in society because we think of them as not being productive, as perhaps being a little bit lazy. And we all become owl-like or more owl-like in our teenage years, too. And I think that's very difficult because parents will often, you know, pull the covers off the teenager at weekends and say, you know, it's daylight out; it's noon; you're wasting the day.

And that's wrong for two reasons. First, the - again, it's not their fault. It's their biology that wants them to be asleep at that time. But it's more than that because it also turns out that they are trying to sleep off a debt that we have actually saddled them with by way of this incessant model of early school start times. And we have to abandon that attitude. And we also have to change the educational practice as well.

GROSS: OK, let's take a short break here, and then we'll talk some more. If you're just joining us, my guest is Matthew Walker. He directs the Center for Human Sleep Science at the University of California, Berkeley. He's the author of the new book "Why We Sleep." We'll be right back. This is FRESH AIR.

(SOUNDBITE OF CARL VERHEYEN'S "GOOD MORNING JUDGE")

GROSS: This is FRESH AIR. And if you're just joining us, my guest is Matthew Walker. He directs the Center for Human Sleep Science at the University of California, Berkeley, and he's the author of the new book "Why We Sleep."

Does the quality of sleep change as you get older, particularly as you get considerably older?

WALKER: It does. And this is usually very depressing news for most of us, and as I am now advancing in my years. And what we know is firstly that the amount of sleep, the total amount of sleep that you get, starts to decrease the older that we get. And I think one of the myths out there is that we simply need less sleep as we age. And that's not true. In fact, we need just as much sleep in our 60s, 70s, 80s as we do when we're in our 40s.

It's simply that the brain is not capable of generating that sleep which it still needs and the body still needs. We also know that the continuity of sleep also starts to fall apart. Sleep becomes much more fragmented. There are many more awakenings throughout the night for a variety of reasons - pain and bathroom trips, et cetera.

But we also know that it's not just the quantity of sleep that changes with aging; it's also the quality of sleep. And it seems to be particularly the deepest stage of sleep, something that we call non-rapid movement sleep or non-REM sleep and the very deepest stages of non-REM sleep. Those are selectively eroded by the aging process. By the time you're in your 50s, you've perhaps lost almost 40 to 50 percent of that deep sleep that you were having, for example, when you were a teenager. By age 70, you may have lost almost 90 percent of that deep sleep.

GROSS: For people who have trouble sleeping, whether it's a function of age or anxiety or pain or just they're not very talented at sleeping, a lot of those people turn to pills of one sort or another to help. So the pill might be an Ambien kind of pill or melatonin or, you know, some other over-the-counter herb or whatever. So let's talk about some of these options and see what you think of them. Let's start with melatonin. I want you to describe what melatonin is and what it's good for and what it's really not going to be very helpful with.

WALKER: Melatonin is actually a hormone that is released when darkness begins. And so during the day, melatonin is actually suppressed and is at low levels within the brain and the body. But as dusk starts to occur, normally melatonin levels will start to rise. And melatonin, as a consequence, has being called the hormone of darkness or the vampire hormone, I think I've even heard people call it. And melatonin is designed to help you regulate the timing of your sleep.

So melatonin does not actually participate in the generation of sleep itself. And it's slightly misunderstood. Many people think that it helps them fall asleep more quickly or stay asleep. That's actually not true if you look at the carefully controlled studies. So perhaps one way to think of melatonin is a little bit like the starting official in the 100-meter race at the Olympics - that official with the gun. The official, melatonin, actually organizes the great sleep race and then begins the race, but that official does not participate in the race itself. And that's the case for melatonin and sleep.

There are a whole set of different chemicals and brain mechanisms that actually generate sleep and get you into sleep. Melatonin simply times when sleep is going to occur, not the generation of sleep itself. I would say, however, that for people who are using melatonin and feel it benefits them then go ahead and keep using it because not to be dismissive, but the placebo effect is the most reliable effect in all of pharmacology. So if you feel as though it's working for you, that's just fine.

GROSS: What is it good for? When should you take melatonin if you're interested in taking it?

WALKER: Melatonin can be useful if you are traveling between time zones. So if you're in a new time zone - let's say that I flew back home to England - and now my body clock is eight hours behind. It's on California time. It's coming up to 11 or midnight in the U.K., but my melatonin rise is not going to start happening for at least another eight hours. Well, I can take some melatonin, and I can fool my brain and body into thinking that it's night time and it's time to sleep. So you can actually use melatonin to help you get back into set, to sort of hit the reset button on your timing of wake and sleep in a new time zone. That's when it's useful.

It can also be useful in older adults who actually have a weak release of melatonin. And they may actually benefit, too, from the use of melatonin regularly in the evening. But overall, for most people, melatonin is not an efficacious sleep aid in the sense of actually helping you fall asleep faster or keeping you asleep for longer.

GROSS: What do you think of prescription sleeping pills? And I'll use Ambien as an example.

WALKER: Unfortunately, the current set or classes of sleeping pills that we have do not produce naturalistic sleep. So they are all a broad set of chemicals that we call the sedative hypnotics. And sedation is not sleep. It's very different. It doesn't give you the restorative natural benefits of sleep. Another concern with sleeping pills is that they have been linked to a higher risk of death and cancer. And I think this evidence has perhaps not made its way clearly out to the public yet.

Now, we don't currently know whether that evidence is simply correlational versus causal. We don't simply know if people who are taking sleeping pills are also people who are more likely to die a faster death or more likely to suffer from cancer. But it could very well be that those sleeping pills do cause a higher likelihood of death and cancer. That data is currently unclear. But I think the public needs to be informed about that evidence.

It also needs to understand that you don't necessarily need to look to sleeping pills to help, particularly if you're suffering from insomnia. There is a non-pharmacological approach that is called cognitive behavioral therapy for insomnia, or CBTI. And you work with a therapist. It lasts a couple of weeks.

And it is just as effective as sleeping pills in the short term. But even better, it lasts once you finish work with a therapist - unlike sleeping pills, where, once you stop them, not only do you tend to go back to the bad sleep that you were having before, but worse, you tend to have what's called rebound insomnia, where your sleep is even worse.

So I wish I could say that the current sleeping medications right now produced beautiful, naturalistic sleep. And I'm not at all against medication. Please don't think that. So it's simply that at this stage, we just don't have that complex medication that can replicate the complex process that is sleep.

BIANCULLI: Matthew Walker, speaking to Terry Gross last year. His book "Why We Sleep" is now out in paperback. After a break, we'll continue their conversation. And film critic Justin Chang will review the films "Blindspotting" and "Sorry To Bother You," two movies set in Oakland. I'm David Bianculli, and this is FRESH AIR.

(SOUNDBITE OF SONG, "I'M SO TIRED")

THE BEATLES: (Singing) I'm so tired. I haven't slept a wink. I'm so tired. My mind is on the blink. I wonder, should I get up and fix myself a drink? No, no, no. I'm so tired. I don't know what to do. I'm so tired. My mind is set on you. I wonder, should I call you? But I know what you would do. You'd say I'm putting you on. But it's so joke. It's doing me harm. You know I can't sleep. I can't stop my brain. You know it's three weeks. I'm going insane. You know I'd give you everything I've got for a little peace of mind. I'm so tired. I'm feeling so upset. Although I'm so tired, I'll have another cigarette and curse Sir Walter Raleigh. He was such a stupid get.

BIANCULLI: This is FRESH AIR. I'm David Bianculli, in for Terry Gross, back with more of Terry's interview with Matthew Walker. He's the author of the book "Why We Sleep," which is now in paperback. Walker directs the Center for Human Sleep Science at the University of California, Berkeley.

(SOUNDBITE OF ARCHIVED BROADCAST)

GROSS: Our culture seems to run on caffeine. You know, like, coffee drinks are so popular, and so are those high-energy drinks that are just like little caffeine drinks. I mean, (laughter) I don't even know how much caffeine is in them. Tell us about how caffeine works in the body.

WALKER: Caffeine is - you're right - it's now, I think, at levels of abuse, we could suggest. It is the second most-traded commodity on the surface of the planet after oil...

GROSS: Are you kidding me? Really?

WALKER: No. And I think that that, to me, is perhaps one of the less scientific but most striking data points demonstrating how sleep deficient we have become throughout developed nations. Caffeine actually works in a way to block the sleepiness signal. So from the moment that both you and I woke up this morning, a chemical has been building up in our brain. And that chemical is called adenosine. And the more of that chemical that builds up, the sleepier we feel. And it creates something that we call sleep pressure.

And after about 16 hours of being awake, we have enough of that sleep pressure to feel tired and fall asleep and stay asleep. Caffeine, on the other hand, actually comes into the brain. And it jumps and latches on to the welcome sites of adenosine in the brain, what we call the receptors within the brain. And it masks those receptors. So after 16 hours of being awake, let's say that you have a cup of coffee, an espresso. And all of a sudden, your brain goes from thinking, I've been awake for 16 hours; I'm tired and sleepy, to then thinking, oh, no. Hang on a second. I haven't been awake for 16 hours at all. I've only been awake for maybe just six or seven hours because the caffeine is blocking that signal of adenosine. It's blocking that sleep pressure instruction to the brain.

The problem, however, with caffeine is - one of the problems - there are many - is that when caffeine is blocking those receptors, the adenosine, the sleepiness chemical, continues to build up. And it continues and it continues so that finally, when your brain gets rid of all of that caffeine out of its system, not only do you go back to that level of sleepiness that you were several hours ago, you're now hit with that level of sleepiness plus all of the additional sleepiness that's been building up in between. And it's what's called the caffeine crash. So now you have to have two espressos rather than just one. And so goes that medication cycle.

GROSS: You also write how contradictory the alcohol response is in terms of sleep. Like, if you're having wine not long before bed, the wine can help put you to sleep, but then it's going to wake you up during...

WALKER: Alcohol...

GROSS: ...The night. Yeah, explain that response.

WALKER: Alcohol in for example wine or whiskey, that sort of classic nightcap, it actually doesn't put you to sleep faster. That's one of the most misunderstood aspects of alcohol. Alcohol, again, is a sedative drug. And what you're doing there is simply knocking yourself out. You are removing consciousness quickly from the brain by way of having alcohol. But you're not putting yourself into naturalistic sleep. The other issue is that alcohol will both fragment your sleep - it will litter it and punctuate it with many more awakenings throughout the night, so short, however, by the way, that you tend not to remember them. And so once again, you're not quite aware of how bad your sleep was when you had alcohol in the system. The final aspect of alcohol is that it is very good at blocking your REM sleep or your dream sleep, which is critical for aspects of mental health within the brain and emotional restitution, too. So alcohol - very misunderstood drug when it comes to sleep. Not helpful.

GROSS: So one of the reasons why you keep telling us we need to be sleeping more, we need to be sleeping eight hours a night, is that sleep helps us remember things. It's good for memory. But it also helps us forget the things that we would just as soon forget (laughter), so...

WALKER: (Laughter).

GROSS: ...How - what goes on in our brain in terms of memory and forgetting as we sleep?

WALKER: Sleep seems to provide a form of forgetting in at least two ways. One is that if information does not seem to be relevant or even if it's duplicate information, sleep will enhance that which is necessary and important to remember. And it will actually let go of that which doesn't seem to be important to you the organism based on a variety of reasons. So sleep can actually not only supply you and facilitate the things that are essential to hold onto, but it will actually clear up storage space, as it were, by removing things that are unnecessary and not wanted so that you don't get clutter. It's like having a wonderful sort of clean desk, a great filing system where only the things that you need are put in place. And they're not sullied by all of the noise surrounding it. There's a very clear signal-to-noise ratio when it comes to information within the brain by way of sleep. So that's one way that sleep helps forgetting.

GROSS: Is the brain too busy while we're awake to do the things that it does while we're sleeping?

WALKER: The brain is certainly caught up with a vast variety of different aspects during waking consciousness. And I think the principal goal when it comes to learning and memory for wakefulness is information reception. We're constantly acquiring, downloading, aggregating that information. In contrast, I think sleep is all about reflection. So rather than it being a state where we're receiving information, it's going back over that information and asking, on the basis of what we learned today, what's novel and new? What do we need to hold onto? What do we need to forget? And how does this new information fit with this vast back catalog of autobiographical information that we have? And how should all of that be stitched together so that when I wake up the next day, I have this improved, accurate statistical model of this thing called the world and how it works so that my survival chances each and every day get better by sleep each and every night?

GROSS: You have people in your sleep lab. And you're imaging their brains as they sleep. When you're doing imaging while somebody's sleeping, are you able to tell when they're dreaming? And are there are times when the person thinks that they haven't had any dreams, but you know that they actually have?

WALKER: Yes on both counts. So we can be sitting in a control room many, many feet away from the individual, and just based on these beautiful brain waves that we can be seeing, we will know with high accuracy not only whether they're awake or asleep and, if they're asleep, what stage of sleep that they're in, we will also be able to predict whether they're dreaming. There is a phase of rapid eye movement sleep that gives it its name where you're actually getting these darting eye movements that go back and forth, left and right, left and right. And if I wake you up as you're having REM sleep, dream sleep, with a phase of these rapid eye movement darts occurring, it's 95 to 100 percent probable that you're going to report a dream when I wake you up. And there are those people who say, I just never dream. I'm just not one of those people who actually dreams. And the answer of course is that you're simply not one of those people who remembers their dreams. Almost everyone dreams. There's a rare type of brain damage that can occur where people seem not to dream, although even that is questionable.

GROSS: If you're just joining us, my guest is Matthew Walker. He's the director of the Center for Human Sleep Science at the University of California, Berkeley, and author of the new book "Why We Sleep." We'll be right back. This is FRESH AIR.

(SOUNDBITE OF JAKE SHIMABUKURO'S "FIVE DOLLARS UNLEADED")

GROSS: This is FRESH AIR. And if you're just joining us, my guest is Matthew Walker, the director of the Center for Human Sleep Science at the University of California, Berkeley, and author of the new book "Why We Sleep."

So one of the areas you've been researching is the correlation between sleep or lack of sleep and Alzheimer's disease. So what have you learned? Is there a correlation that you're finding?

WALKER: There's a strong relationship now between insufficient sleep and the mechanisms of Alzheimer's disease. Sadly, it's a two-way street as well that we've discovered. There was a remarkable finding several years ago that revealed the brain actually has a sewage system inside of it. And our body actually has one. It's called the lymphatic system that many people have heard of. But only recently did we discover that the brain has one. It's called the glymphatic system. And one of the things that that sewage system in the brain does very well is clear out all of the detritus that's been building up, all of the metabolic waste that's been building up throughout the day.

The second discovery, though, is that the sewage system in the brain actually kicks into high gear when we go into sleep and particularly deep sleep. And one of the toxic byproducts that it actually removes at night is a sticky protein called beta amyloid, which is one of the leading causes that - we believe - underlying Alzheimer's disease.

So firstly, we start to understand now why short sleep seems to predict a far higher risk of developing Alzheimer's disease. And it does. That data now from epidemiological studies is there, and I think it's quite clear. But now we're starting to understand exactly why insufficient sleep is one of the most significant lifestyle factors determining whether or not you will develop Alzheimer's disease.

The two-way street, sadly, is that if that toxic, sticky protein, beta amyloid, builds up in the brain, it doesn't simply build up everywhere in the brain constantly. It seems to build up in some regions more quickly and more severely than others. And sadly, the regions that it targets very early on and aggressively are the deep sleep-generating regions.

So you produce this vicious cycle. If you're not getting enough sleep throughout your life, more of this protein seems to build up in the brain. The more of that protein that builds up, the harder it is to actually get that deep quality of sleep that you need whenever you do get the chance to get sleep. So the less of that toxic protein you're able to wash away at night, the more of that toxic protein that builds up, the harder it is to fall asleep, so on and so forth. It's this ongoing self-fulfilling prophecy.

GROSS: Have you followed up with people who do have Alzheimer's and investigated whether they were good sleepers or not and whether that correlation seems to bear out in people who already have Alzheimer's?

WALKER: That's exactly what we're doing right now. We've been fortunate to receive grant funding to actually really dig down and do a deep dive into this issue. We are doing retrospective analyses to try and see if we can understand how people were sleeping in midlife to see if that is a deterministic factor. We are also starting to explore the sleep of patients who have quite severe pathology in their brain, quite severe stages of Alzheimer's disease within the brain.

We already know that patients with Alzheimer's disease have remarkably bad sleep. It's far worse than even someone of the same age counterpart but who does not have Alzheimer's disease. We know that as sleep gets worse in Alzheimer's patients, it walks and strides in lockstep with the decline cognitively of those Alzheimer's patients, too. So those two things seem to go hand in hand. And perhaps one - perhaps - is causing the other. And we're going to try and understand that, too.

I think one of the other aspects, though, to give perhaps a little glimmer of hope is that we're starting to try and explore therapeutic avenues. If sleep is causally instrumental, if sleep is a missing piece in the explanatory puzzle of aging and Alzheimer's, it's exciting because we may be able to do something about it. And here we're not talking about sleeping pills again just because they don't produce that good, healthy sleep. We're actually starting to develop brain stimulation technology where we're actually trying to sing in time with these deficient deep sleep brain waves, almost like acting as if we were a choir to a flagging lead vocalist. And we're trying to amplify the size of those deep sleep brain waves. And in doing so, we're going to try and salvage aspects of learning and memory function that depend on that deep sleep.

And if that works, maybe we can actually shift that treatment benefit from an end-stage model of therapy, of treatment, to a midlife intervention which is actually prevention rather than treatment. So it's a lofty goal. It's one of my moonshots. And I'm sure we will likely fail, but we are definitely going to try. We have to try.

GROSS: I certainly hope you have success with that research. I'd like to ask you to sum up for us some of the things we can do to improve the quantity and quality of our sleep. Judging from what you've said, we should be sleeping in a dark room because light interferes with sleep. Stay away from screens. We all know we're supposed to do that whether we do it or not.

(LAUGHTER)

GROSS: What else can we be doing to make sure that we fall asleep and stay asleep?

WALKER: You've hit on 1 of I think 5 tips for getting better sleep right now. The first is dark. We are a dark deprived society. So stay away from those screens and laptops as you've said, but also dim the lights down in the evening. We don't need all of the lights blazing in the last hour or so before bed. That will help, and blackout curtains can be of use, too. The four other tips would be this - regularity. If there is one piece of advice that folks should follow for better sleep, it is this - go to bed at the same time. Wake up at the same time no matter whether it's the weekday, the weekend - whether you've had a good night of sleep or bad night of sleep.

The second is temperature, and keep it cool at night - cooler than you think. Probably around 65 to 68 degrees, which I know sounds too cold. But your body needs to drop its temperature to initiate sleep, and it's the reason it's always easier to fall asleep in a room that's too cold than too hot. So keeping it cool moves your body in the right direction for good sleep.

The other thing we've spoken about - don't stay in bed if you've been awake for the reasons we suggested because your brain learns the connection between wakefulness and your bed. So get out. Go to a different room. The final aspect we've already discussed as well, which is no caffeine or alcohol. Sorry to plug that again, but it will really do you a remarkable disservice by loading with caffeine sometime after 2 p.m. or having that nightcap in the evening. Both should be avoided if you wish for good sleep.

GROSS: I'm sure you've seen all the ads for those dual mattresses you can adjust like - you can adjust one side of the bed. And your partner or your spouse can adjust the other side of the bed differently (laughter). So the premise of all these mattresses and of the commercials is, gosh, it's really hard to share your bed with anybody and still get a good night's sleep. Here's one solution.

(LAUGHTER)

GROSS: So in your experience, is sleeping with a partner or a spouse contraindicated for a good night's sleep?

WALKER: Not always. Some people actually sleep better when the significant other is with them. For other couples, it's the opposite.

GROSS: I think that's true. Some people can't sleep at all - right - if the partner's not there. Yeah.

WALKER: That's right. But for others, it goes a different route. And we know that about 30 percent of people seem to sleep in separate rooms. We also know that of those who remain or who go to sleep together, somewhere between 20 to 40 percent admit to waking up in different locations. So some (laughter) have described it as a sleep divorce, which I think is desperately negative.

GROSS: (Laughter).

WALKER: And I don't want to say that. But if it leads to better sleep, I'm all for it. We also know that people who are getting sufficient sleep have a far healthier physical relationship. We know that men, for example, who are sleeping just five to to six hours a night will have a level of testosterone which is that of someone 10 years their senior. So it ages you by a decade in terms of that aspect of health and virility, and we see similar impairments in female reproductive health with insufficient sleep.

GROSS: OK, think I'll go to sleep.

(LAUGHTER)

GROSS: Thank you so much for talking with us.

WALKER: You're so very welcome.

BIANCULLI: Matthew Walker spoke to Terry Gross last year when his book "Why We Sleep" was published. It's now out in paperback. Walker directs the Center for Human Sleep Science at the University of California, Berkeley. Coming up, film critic Justin Chang reviews "Blindspotting" and "Sorry To Bother You," two new films which are each set in Oakland and are noted for their interesting takes on race. This is FRESH AIR.

(SOUNDBITE OF ALLEN TOUSSAINT'S "BRIGHT MISSISSIPPI") Transcript provided by NPR, Copyright NPR.